When he says "pull your hips back" (which would help) that also means that your weight will have to be on your heels... so when you are setting up and feeling the tension in your hamstrings (I've never heard it described like that but its bang on, if you take too long it actually starts to burn) make sure you are sitting back on your heels and the bar is still touching your shins.

rippetoe says to line up with the bar cutting your feet roughly in half when you look down. when you squat down into position, the bar should touch your shins. this means your scalpula is directly overhead, and you won't have to compensate at all by bringing the bar into or away from your body (you can't do the lift if the bar is not eventually perpendicular to your scalpula).

I guarantee you will shoot up in weight if you practice, 275 will feel like nothing.